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In this work, we report a structurally-colored multilayer film made of a silk-based material, inspired by the cuticle of Hoplia coerulea, a species of beetle able to change its color in the presence of moisture. For the generation of interference color, we combined the regenerated silk fibroin, which has a refractive index of 1.55, with a nanocomposite made of silk and titanate nanosheets, a novel 2D material which presents a high refractive index. The multilayer film was fabricated through a layer by layer deposition from the respective water-based solutions by spin coating. The final structure exhibits an interference peak in the transmittance spectrum centered at 400 nm, whose position responds to changes in the relative humidity of the environment. The stimuli-responsive properties of the film were characterized and a simple optical model for the sensing mechanism was proposed. Specifically, the silk-based multilayer showed a reversible color change when exposed to different relative humidities, and good performances in terms of reproducibility and stability over time.

The scales covering the elytra and thorax of the male Hoplia coerulea scarab beetle are a specific case of natural photonic structure encased by a fluid-permeable envelope. These scales display several optical effects including structural colouration, fluid-induced colour changes, fluorescence emission and fluid-induced fluorescence changes. Although the photonic structure is not directly exposed to the environment, optical changes are known to be induced by liquids and vapours. These changes are controlled by the envelope that mediates exchanges with the surrounding environment. The optical system comprising the photonic structure and the envelope was previously termed “photonic cell”. Novel possibilities to develop functional photonic materials and smart coatings are offered by such a system through a bioinspired approach. This technological potential motivated us to analyse theoretically, through numerical simulations, the water-induced changes of optical properties of light emitters embedded in a photonic structure inspired by H. coerulea beetle’s scales.

Porous photonic structures found in several living organisms are known to display colour changes induced upon contact with liquids, vapours and gases. Usually these changes are due to physico-chemical phenomena such as the swelling of the structure enacted by the fluids, vapour physisorption on the pore walls, capillary condensation or a combination of them. Generally, the porous structures are open to outside, leading to fast fluid exchanges with the surrounding environment and consequently fast colour changes. In this article, we first introduce fluid-induced optical changes in living organisms exhibiting porous photonic structures. We explore then the potentiality of environmental ellipsometry for the first time in the context of natural photonic structures through the investigation of the optical response of the male cerulean chafer beetle Hoplia coerulea (Scarabaeidae) upon contact with water, 2-propanol and toluene vapours. In contrast with most of the investigated photonic structures, this beetle’s structure is encased by an envelope that mediates liquid exchanges with the environment. Such a study is of great interest in order to understand the underlying biological functions behind these changes as well as in order to develop bioinspired applications such as gas sensors and other environment-responsive coatings

Se presenta una discusión acerca del fenómeno de iridiscencia presente en los élitros del Euchroma gigantea, insecto que pertenece a la familia Buprestidae, los denominados escarabajos joya. Las ondas reflejadas sugieren que el fenómeno de iridiscencia es estructural, justo como se manifiesta en las mariposas Morpho o en los escarabajos Hoplia coerulea. Las imágenes obtenidas por SEM muestran una la estructura superficial de los élitros como un arreglo aleatorio de defectos en un sustrato dieléctrico orgánico.Como el componente principal del exoesqueleto de los artrópodos es la quitina, se hace la suposición que el sustrato es completamente quitina y también se asume que los defectos son columnas de aire. Es así como el sistema es modelado como un cristal fotónico dos-dimensional. Debido a que las energías características obtenidas para los modos activos de la estructura no pertenecen al rango visible, se concluye que el fenómeno de iridiscencia no es causado por la estructura superficial del élitro. In this work we present a discussion about the iridescence phenomena exhibited on Euchroma gigantea’s elytra (1), a beetle which belongs to the family of Buprestidae, the jewel beetles. The reflectance suggest that iridescence phenomena is structural, the same as Morpho butterflies or the Hoplia coerulea beetles. The SEM images reveals a superficial structure of the elytra, as an aleatory arrange of defects in a dielectric organic substrate. As the main component on exoskeletons of arthropods is the chitin, we suppose the elytra are completely made of chitin and also is assumed the defects are air columns. That is how the system is modeled as a two dimensional photonic crystal.Because of the characteristics energies of each active mode calculated for the estructure doesn’t belong to visible range, is concluded that the iridiscence phenomena is not caused by its superficial structure.

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as non-linear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein these techniques are applied for the first time to the study of insects' natural fluorescence. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's colouration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A Third-Harmonic Generation signal is also detected, the intensity of which depends on the light polarisation state. The analysis of these non-linear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The anisotropy of the photonic structure, which causes additional tailoring of the beetle's optical responses, is confirmed by circularly polarised light and non-linear optical measurements., Comment: 12 pages, 4 figures, 1 table, 1 SI file

Natural nanostructures rarely come with a single biological function to fulfil. Moreover, from a bio-inspiration perspective, it sounds attractive to develop multifunctional coatings, devices or sensors. Suppression of light reflection from body parts, such as the wings of insects, is useful for hiding from predators. The transparent parts of the wings of Cacostatia ossa (moth) inherit their antireflective property from non-close-packed nano-scale nipple arrays on both sides of the wings. Through modelling and optical simulations, we show that effective medium approaches, commonly used to characterize antireflection, slightly overestimate the reflectance with respect to detailed Rigorous Coupled Wave Analysis calculation. Coloration due to light interference in nanostructure, on the other hand, sometimes comes with an additional, unexpected and maybe non-biologically significant function: hygrochromism, i.e. change of color with humidity. The male Hoplia coerulea (beetle), for instance, exhibits iridescent blue-violet color which turns to emerald green when the elytron is put in contact with water. Impregnation experiments with various liquids revealed intriguing color change dynamics which could be related to the wetting properties of porous chitinous cuticle. Super-hydrophobicity is another function of biological significance, which helps, for instance, insects to keep dry in humid environments. Through morphological, optical and contact angle measurements, we show the existence of entangled levels of functionality on the wings of Cicada orni, namely a superhydrophobic stage at the upper part of the nipple array corrugated surface and an antireflective stage and the lower part. Natural nanostructures rarely come with a single biological function to fulfil. Moreover, from a bio-inspiration perspective, it sounds attractive to develop multifunctional coatings, devices or sensors. Suppression of light reflection from body parts, such as the wings of insects, is useful for hiding from predators. The transparent parts of the wings of Cacostatia ossa (moth) inherit their antireflective property from non-close-packed nano-scale nipple arrays on both sides of the wings. Through modelling and optical simulations, we show that effective medium approaches, commonly used to characterize antireflection, slightly overestimate the reflectance with respect to detailed Rigorous Coupled Wave Analysis calculation. Coloration due to light interference in nanostructure, on the other hand, sometimes comes with an additional, unexpected and maybe non-biologically significant function: hygrochromism, i.e. change of color with humidity. The male Hoplia coerulea (beetle), for instance, exhibits iridescent blue-violet color which turns to emerald green when the elytron is put in contact with water. Impregnation experiments with various liquids revealed intriguing color change dynamics which could be related to the wetting properties of porous chitinous cuticle. Super-hydrophobicity is another function of biological significance, which helps, for instance, insects to keep dry in humid environments. Through morphological, optical and contact angle measurements, we show the existence of entangled levels of functionality on the wings of Cicada orni, namely a superhydrophobic stage at the upper part of the nipple array corrugated surface and an antireflective stage and the lower part.

H. coerulea’s elytra show a strong two-photon fluorescence signal, which depends on the presence of water in the photonic structure. A polarization-dependent Third Harmonic Generation signal was detected as well and thoroughly investigated.

The structural colour of male Hoplia coerulea beetles is notable for changing from blue to green upon contact with water. In fact, reversible changes in both colour and fluorescence are induced in this beetle by various liquids, although the mechanism has never been fully explained. Changes enacted by water are much faster than those by ethanol, in spite of ethanol’s more rapid spread across the elytral surface. Moreover, the beetle’s photonic structure is enclosed by a thin scale envelope preventing direct contact with the liquid. Here, we note the presence of sodium, potassium and calcium salts in the scale material that mediate the penetration of liquid through putative micropores. The result leads to the novel concept of a “photonic cell”: namely, a biocompatible photonic structure that is encased by a permeable envelope which mediates liquid-induced colour changes in that photonic structure. Engineered photonic cells dispersed in culture media could revolutionize the monitoring of cell-metabolism.

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Photonic structures encased by a permeable envelope give rise to iridescent blue color in the scales covering the male Hoplia coerulea beetle. This structure comprises a periodic porous multilayer. The color of these scales is known for changing from blue to green upon contact with water despite the presence of the envelope. This optical system has been referred to as a photonic cell due to the role of the envelope that mediates fluid exchanges with the surrounding environment. Following from previously studied liquid-induced changes in the color appearance of H. coerulea, we measured vapor-induced color changes in its appearance. This response to vapor exposure was marked by reflectance redshift and an increase in peak reflectance intensity. Different physico-chemical processes were investigated to explain the increase in reflectance intensity, a property not usually associated with vapor-induced optical signature changes. These simulations indicated the optical response arose from physisorption of a liquid film on the beetle scales followed by liquid penetration through the envelope and the filling of micropores within the body of the photonic structure.

The blue scales on the cuticle of the male beetle Hoplia coerulea can absorb water, with the consequence that these scales, which have been shown to be responsible for the beetle's bright blue coloration, reversibly turn to emerald green with increasing water contents. Optical measurements are shown, by analytic photonic-crystal models, to be compatible with the full filling of the scales structures with water. The natural mechanism shows the way to produce a very efficient hygrochromic material: a medium which significantly changes color when its water contents are modified.

The male of the beetle Hoplia coerulea is known for its spectacular blue-violet iridescence. The blue coloration is caused by the presence of an interesting photonic structure inside the scales which cover the dorsal parts of the insect's body. This structure can be described as the stacking of chitin plates supporting arrays of parallel rods. The change of colour of this structure with humidity is investigated, as well as its response to some other external conditions, such as mechanical strain.

Hoplia coerulea is known for its spectacular blue-violet iridescence. The blue coloration is caused by the presenceof a photonic structure inside the scales which cover the dorsal parts of the insects body, including the head,the thorax, and the wing cases. The structure can be described by a stack of chitin plates wearing arrays ofparallel rods. This arrangement leads to a multilayer structure which only uses a single solid material. The shiftof the re”ected wavelength to the ultraviolet (passing through violet iridescence) is described and explained onthe basis of the optical properties of this structured metamaterial.Keywords: Re”ectance, natural photonic structures 1. INTRODUCTION This paper deals with the origin of the blue re”ectance from the Hoplia coerulea (Coleoptera), a beetle whichbelongs to the large family of Scarabaeidae. It has long been known that metallic coloration of birds andinsects was often due to structural frequency selection, a fact which was already questioned in 1911 in a paperby Michelson.

The microscopic structure of the hard external parts of the body of the iridescent blue-violet chaffer beetle Hoplia coerulea is studied using scanning electron microscopy. The blue iridescence is shown to originate from the structure of the squamae within scales covering the dorsal side of the beetle. The internal structure of the scales shows a stack of planar sheets, separated by a well-organized network of spacers, a structure which belongs to the family of the layer-by-layer photonic crystals. The blue iridescence is easily explained by a planar multilayer approximation model, deduced from the observed three-dimensional structure.

International audience; Scales of the scarabaeid beetle Hoplia coerulea (Coleoptera) contain fluorescent molecules embedded in a multilayer structure. The consequence of this source confinement is a modification of the fluorescence properties, i.e., an enhancement or inhibition of the emission of certain wavelengths. In this work, we propose a bio-inspired approach to this problem. In other words, we use numerical simulations based on the one-dimensional transfer-matrix formalism to investigate the influence of a Hoplia-like system on emission characteristics and, from the results, we deduce potential technical applications. We reveal that depending on the choice of some parameters (layer thickness, dielectric constant, and position of the emitting source in the structure), it is possible to enhance or inhibit the fluorescence emission for certain wavelengths. This observation could be of great interest to design new optical devices in the field of optoelectronic, solar cells, biosensors, etc. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768896]